ActivityQueue is too slow

When creating a bunch of activities, the ActivityQueue take something from 10 to 60 seconds before actually send the activity.

To reproduce:

1. Create a DjangoLDP server following the Hubl format

• You may want to clog the Prosody's part, you can emulate it with this node server using fastify+fastify-plugin. If you use it, you'll need to put the value of JABBER_HOST from the js file on the JABBER_DEFAULT_HOST of the template and http://localhost:4848 as PROSODY_HTTP_URL.

2. Create an RSA Key ./manage.py creatersakey
3. Register an administrator ./manage.py createsuperuser
4. Register default subscriptions ./manage.py create_subscription

Now, create an user, a circle, or save any resource related to DjangoLDP-Account, DjangoLDP-Circle, DjangoLDP-Project and go to http://localhost:8000/admin/djangoldp/activity/ notice how much time it gets to send any activity and in how much time you'll get something on the node server.

@balessan @calummackervoy

Related infra/prosody-modules#17 (comment 52564)